Definition of know what one is talking about in English:

know what one is talking about

phrase

  • Be expert or authoritative on a particular subject.

    ‘I know what I'm talking about—I've built up three businesses from scratch’
    • ‘But why on earth, before one knows what one is talking about and without the scientific evidence to make a judgment, would one nevertheless make that judgment?’
    • ‘Unless one is in that situation and really knows what it is like to face those sorts of family difficulties, one does not know what one is talking about when pontificating about cultural sensitivities.’